1. Field of the Invention
The system in accordance with the invention is an acoustic imaging system which enables images in two dimensions to be obtained. In particular, such a system can be used to display underwater objects.
2. Description of the Prior Art
Acoustic imaging finds many applications in the fields of medecine, non-destructive testing and also the underwater acoustic field. Generally these imaging systems transmit acoustic energy in the form of ultrasonic waves which fill the surrounding space. This energy is returned by reflection to reception devices which, after various processes, supply an image which is similar in appearance to an optical image.
It is known that, as reception means, an array formed of transducers supplying electrical signals can be used, these signals being processed to obtain a signal representing the acoustic energy coming from one direction, this signal then being detected before being transmitted to a display device.
It is also known that the signals coming from transducers spaced periodically in a linear array can be processed by performing the Fourier transform of a signal formed by all the signals in successon from the transducers taken in a very small interval of time compared with the reciprocal of the apparatus pass band. However, this method of processing is only possible if the frequency band of the signal transmitted is small compared with the high frequency carrier.
Finally, it is known that the Fourier transform of an analog signal can be produced by means of filters which are dispersive to elastic waves, such a device being known as a "chirp transform".
The image system proposed contains in particular a flat reception array consisting of periodically spaced transducers in which processing of the signals coming from the transducers is done by performing the Fourier transform of means of elastic wave dispersive filters for surface or volume waves.
In all imaging systems, the spatial resolution, i.e. the distance for which two points in the image plane are separable, is determined by the dimension of the reception array expressed in wavelengths.
A technique, which is often used in the radar field, consists in synthesizing a bigger array dimension by using the movement of the transmitter and reception array with respect to the object.
Another technique, described in U.S. Pat. No. 4,119,940 by Keating, consists in using several transmitters separated in space whose positions are known with respect to the reception array, each transmitter supplying a coded signal so that it can then be separated out at the level of each transducer in the reception array. The signals are then converted into digital samples to be processed by a computer which reconstructs the image by carrying out in succession the focussing and Fourier transform operations. This system, which uses an acoustic holography technique, as the disadvantage of imposing a low limit on the time required to obtain an image because of the time taken for the calculations in the computer even when the best algorithms of the fast Fourier transform, called F.F.T, are used, whereas the display of mobile objects moving rapidly requires that this time be reduced to obtain clear images.
The system in accordance with the invention enables an image of the same dimensions to be obtained in a shorter time by the use of analog devices.
In underwater acoustics, apparatuses for sonar imaging of sea bottoms of the side-scan or frontal types have long been used. They contain a reception array and a transmission array and the intention is to supply an image of the sea bottom in order to detect and identify objects which may be located there (mines for example). In these systems, the signals coming from the reception array are spatially processed to give incident beams which are oblique with respect to the bottom. The image supplied at each instant consists of lines formed by image "points" corresponding to the directions of the beams formed, each line corresponding to a distance.
To improve the range resolution, the acoustic signal is transmitted with a band B and an image line must be obtained within a time interval less than or equal to the reciprocal of this band. Hence the advantage in using fast technologies. For this purpose, in accordance with prior art, the beam formation is obtained in analog fashion by using delay lines with multiple connections, placed behind each reception array transducer, and a set of adders. The signal corresponding to a beam is obtained at the output of each adder. Because of the resolution several dozen beams are formed and the volume of electronic circuitry is very large.